I too have had problems with COBRA's autopay. A representative even said they have a problem with autopay especially when something changes with the autopay details. It happened when I first set it up in April.
And then again in October, I had to change from my HSA card (exhausted) to a regular credit card for my payments in Nov and December 2018. The autopay didn't work for Nov so I paid it and made sure autopay was set for December.
I received a notice in January 2019 that they terminated my coverage December 1, 2018 due to nonpayment. I requested for them to let me pay the December payment and they denied the request. I even showed my account that signified it was on autopay.
I had many medical services in December and am now on the hook for over $3,000 in medical fees. Is there anything I can do?